This invention relates to ultrasonic transducers in general and more particularly to an ultrasonic transducer arrangement with a matrix of ultrasonic oscillators consisting of several acoustically separated transducer elements which are electrically controlled together.
An ultrasonic transducer arrangement of this general nature is disclosed in German Pat. No. 28 29 570. In nondestructive material testing, images from the interior of a body to be examined are produced by means of ultrasonic pulses which are emitted by a transducer element arranged at the surface of the body. From the travelling time of the ultrasonic signal and the echo signal, the location of a fault can be derived. The ultrasonic transducer arrangement in the form of a so-called array consists of a multiplicity of ultrasonic oscillators with transducer elements of piezo material which are arranged at a close spacing of, for instance, about 50 to 70 .mu.m side by side. The transducer elements are controlled jointly. The entire array may consist, for instance, of about 54 ultrasonic oscillators which are divided by so-called fine division into several transducer elements which are electrically controlled together. By means of this fine division, the transversal vibration of the transducer elements which is also emitted is shifted to higher frequencies and its influence on the resolution is thereby reduced accordingly. Several oscillators of the array can be combined in an oscillator group.
This fine division of the ultrasonic oscillators in the longitudinal direction is in general accomplished mechanically by sawing. Since the height of the transducer elements must not substantially exceed one-half the wavelength of the ultrasonic pulses, the height of the transducer element is also limited accordingly for higher frequencies, for instance, above 10 MHz. The width of the saw gap between the separating surfaces of the transducer elements, however, cannot fall below a predetermined value since sufficient mechanical strength of the saw blades must be ensured. Through this increase of the gap width relative to the areas, the cutting losses are increased accordingly. The radiation per unit area is thereby reduced.
For the operation of ultrasonic equipment for producing what are called B displays in the frequency range of about 2 to 8 MHz, four linear arrays are used as ultrasonic antennas. The arrangement of the oscillators in the longitudinal direction of the array, which is, at the same time, the scanning direction of the emitted ultrasonic pulses, makes possible electronic focusing of the ultrasonic pulses through propagation time delay. Focusing perpendicularly to the scanning direction is possible only with a mechanical focus, for instance, by placing an acoustical cylindrical lens in front. A fixed frequency is assigned to the individual arrays. With mechanical focus, however, adjustment to different depths is possible only at relatively high costs. For operation with different frequencies, several arrays are threrefore also necessary.
To obtain better imaging conditions, mixed operation with two frequencies has already been carried out with linear arrays. In a known device, the compromise between sensitivity and bandwidth has been circumvented through the use of two arrays for different frequencies, for instance, 1.5 MHz and 2.5 MHz, which are arranged side by side (IEEE Transactions on Sonics and Ultrasonics, vol. SU-25, no. 6, Nov. 78, pages 340 to 345). Also in this device, a fixed frequency is assigned to each individual array.
It is therefore an object of the present invention to provide an ultrasonic transducer arrangement, the frequency of which is freely selectable within a certain range and with which better imaging conditions, particularly increased resolution, are obtained in the production of images of a scanned space. With a special embodiment of the arrangement, electronic focusing in the longitudinal direction, as well as in the transverse direction of the arrangement, should also be possible.